Cellular physiology may be regulated by a variety of mechanisms, originating both inside and outside the cell. In multicellular organisms, these mechanisms may involve cell-signaling pathways in which signal substances are released by one cell to influence the position, nature, and activity of other cells.
FIG. 1 is a schematic view of a representative cell-signaling pathway 100. Here, signaling cells 102 produce signal substances 104a,b that interact with target cells 106 to effect a response in the target cells. These responses may be short term, such as glycogen breakdown or muscle contraction, among others. These responses also may be long term, such as growth, differentiation, reproduction, and/or apoptosis, among others. Generally, these responses are brought about by increasing, decreasing, or maintaining enzyme activity in the target cells.
Signaling cells 102 are cells capable of producing a signal (substance) that can effect a specific response in another (target) cell. The signaling cells may be components of an endocrine, paracrine, or nervous system. The endocrine system is an organism-wide control system that regulates body function using hormones released by endocrine organs into the bloodstream. The endocrine organs include the pituitary gland, thyroid gland, parathyroid glands, adrenal glands, thymus gland, pineal body, pancreas, ovaries, testes, and kidneys. The paracrine system is a local control system that regulates nearby cells using local mediators released into the extracellular medium. The nervous system is a specialized control system that regulates specific cells using electrical impulses and neurotransmitters.
Signal substances 104a,b are substances through which a signaling cell may communicate with target cells, evoking a specific response. Signal substances may act as hormones, local mediators, and/or neurotransmitters, among others. Signal substances may take the form of proteins, small peptides, amino acids, nucleotides, steroids (e.g., cortisol, steroid sex hormones, vitamin D), retinoids, fatty acid derivatives, and dissolved gases (e.g., nitric oxide (NO) and carbon monoxide (CO)), among others.
Target cells 106 are cells capable of responding to a specific signal substance produced by a signaling cell. The ability to respond may depend on the cell and on the signal substance. For example, the signal substance thyroxine from the thyroid gland may evoke a response in nearly all cells, whereas the signal substance progesterone from the ovary may evoke a response only in specific cells in the lining of the uterus. The target response may include kinase activity, GTP binding, and/or cyclic nucleotide production.
The ability of a cell to respond to a given signal substance generally is determined by whether the cell includes a receptor for the signal substance. Here, a receptor is any molecule or supramolecular assembly capable of specifically binding a signal substance and initiating a response in a target cell. Representative receptors include cell-surface receptors 110 located on the surface of the target cell and intracellular receptors 112 located within the cytosol 114 or nucleus 116 of the target cell.
The nature of the response initiated by binding of a signal substance is determined by the intracellular machinery to which the receptor is operatively coupled. For example, binding of the neurotransmitter acetylcholine to identical receptors in heart muscle cells and secretory cells causes muscle relaxation in the heart muscle cells and secretion in the secretory cells, due to differences in the associated intracellular machinery.
The remainder of this section examines (1) the receptor mechanisms that cells use to bind signal substances and to communicate this binding to the cell interior, (2) the intracellular pathways that cells use for regulation, and (3) the effects of errors in cell-signaling pathways.
1. Receptor Mechanisms
Target cells generally have receptors capable of specifically binding specific signal substances, including cell-surface receptors and/or intracellular receptors, as described above. Cell-surface receptors are more common and include (A) G-protein-linked receptors, (B) enzyme-linked receptors, and (C) ion-channel-linked receptors. These receptors typically bind large and/or water-soluble signal substances, such as many peptide hormones. Intracellular receptors are less common and include (A) guanylyl cyclase and (B) ligand-activated gene regulatory proteins. These receptors typically bind small and/or water-insoluble signal substances, such as steroid hormones, thyroid hormones, retinoids, vitamin D, and NO.
FIG. 2 is a schematic view of a representative G-protein-linked cell-surface receptor mechanism 130 that includes a receptor protein 132, a G-protein 134, and a target protein 136. These proteins may be positioned on or within the plasma membrane 138 of a target cell. In use, a specific signal substance 140 binds to a signal-substance binding site 142 on the extracellular side 144 of the receptor protein and thereby creates, exposes, or otherwise activates (*) a G-protein binding site 146 on the intracellular side 148 of the receptor protein. The G-protein then binds to the G-protein binding site on the receptor protein and thereby creates, exposes, or otherwise activates (*) a target-protein binding site 150 on the G-protein. The G-protein then dissociates from the receptor protein, binds (via the target-protein binding site) to the target protein, and activates (*) the target protein. Activation and deactivation of the G-protein may involve binding of a guanosine triphosphate (GTP) molecule and dephosphorylation of the GTP molecule, respectively. The receptor protein may belong to a large superfamily of homologous, seven-pass transmembrane proteins. These seven-pass proteins consist of a single polypeptide chain that crosses the membrane seven times, with an extracellular signal-substance binding portion and an intracellular catalytic portion. The G-protein may be trimeric, consisting of three polypeptide chains α, β, and γ—that associate and dissociate during signaling. The target protein may consist of an enzyme or ion channel, among others. In particular, the target protein may be an enzyme that modulates the presence or activity of second messengers within the cell. These second messengers (also known as intracellular messengers or intracellular mediators) may bind allosterically to specific cellular proteins to alter their conformation and hence their activity. These second messengers include adenosine 3′,5′-cyclic monophosphate (cAMP) and calcium (Ca2+).
In the cAMP pathway, the target protein may be adenylyl cyclase (also known as adenylate cyclase), and the G-protein may be a stimulatory G-protein (Gs) that activates the adenylyl cyclase to make cAMP, or an inhibitory G protein (G) that inhibits the adenylyl cyclase to prevent it from making cAMP. The cAMP produced by the adenylyl cyclase activates cAMP-dependent protein kinase (A-kinase), which is a serine/threonine kinase that in turn activates or inhibits other enzymes to effect a physiological response. For example, in connection with glycogen metabolism, A-kinase may inhibit glycogen synthase to shut down glycogen synthesis, and simultaneously activate phosphorylase kinase that in turn activates glycogen phosphorylase to break down glycogen. A variety of signal substances use cAMP as a second messenger, including calcitonin, chorionic gonadotropin, corticotropin, epinephrine, follicle-stimulating hormone, glucagon, luteinizing hormone, lipotropin, melanocyte-stimulating hormone, norepinephrine, parathyroid hormone (PTH), thyroid-stimulating hormone, and vasopressin. The level of cAMP is reduced by phosphodiesterases, and the activity of kinases is reversed by phosphatases, as described below.
In the Ca2+ pathway, the target protein may be a phospholipase with specificity for a phosphoinositide (i.e., inositol phospholipid), and the G-protein may be Gq, which activates the phospholipase to cleave the phosphoinositide to produce an intermediate that releases Ca2+ from the endoplasmic reticulum. For example, the phospholipase phosphoinositide-specific phospholipase C (phospholipase C-3) cleaves the phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) to produce the second messengers inositol triphosphate (IP3) and diacylglycerol. The inositol triphosphate is water soluble and diffuses to the endoplasmic reticulum (ER), where it releases Ca2+ from the ER by binding to IP3-gated Ca2+-release channels in the ER membrane. The diacylglycerol is membrane bound and may be cleaved to form the second messenger arachidonic acid or may activate the Ca2+-dependent serine/threonine kinase protein kinase C that in turn activates or inhibits other enzymes to effect a response. A variety of signal substances use Ca2+ as a second messenger, including acetylcholine, antigen, thrombin, and vasopressin.
FIG. 3 is a schematic view of a representative enzyme-linked cell-surface receptor mechanism 170 that includes a receptor protein 172 positioned across the plasma membrane 174 of a target cell. The receptor protein includes a signal-substance binding site 176 on the extracellular side 178 of the membrane and a catalytic portion 180 on the intracellular side 182 of the membrane. (In some cases, the catalytic portion of the receptor may be replaced or augmented by a separate enzyme directly associated with the receptor protein.) In use, a specific signal substance 184 binds to the signal-substance binding site, initiating a series of events (such as dimerization and concomitant autophosphorylation of the receptor proteins) that activates (*) the catalytic portion of the receptor. The receptor protein may belong to one of at least five classes of single-pass transmembrane proteins: (A) receptor guanylyl cyclases, which catalyze the production of guanosine 3′,5′-cyclic monophosphate (cGMP) in the cytosol; (B) receptor tyrosine kinases, which phosphorylate specific tyrosine residues on some intracellular proteins, (C) tyrosine-kinase-associated receptors, which associate with proteins that phosphorylate specific tyrosine residues on some intracellular proteins; (D) receptor tyrosine phosphatases, which dephosphorylate specific tyrosine residues on some intracellular proteins, and (E) receptor serine/threonine kinases, which phosphorylate specific serine or threonine residues on some intracellular proteins. Some of these receptors are described below in more detail.
The signal substance also may bind to intracellular receptors, such as guanylyl cyclase. This enzyme produces cGMP from GTP, which then acts as a second messenger much like cAMP. As described above, cGMP also may be produced by enzyme-linked cell-surface receptors. cGMP is present in most tissues at levels 1/10 to 1/100 those of cAMP. A variety of compounds increase cGMP levels in cells, including (1) the hormones acetylcholine, insulin, and oxytocin, (2) the guanylate cyclase stimulators (and vasodilators) nitroprusside, nitroglycerin, sodium nitrate, and nitric oxide, (3) chemicals such as serotonin and histamine, and (4) peptides such as atrial natriuretic peptide (ANP) that relax smooth muscle.
2. Intracellular Signaling Pathways
Target cells may have intracellular signaling pathways capable of specifically binding signal substances, including cell-surface receptors and intracellular receptors, as described above. These pathways may include (1) a phosphorylation pathway involving ATP/ADP, and (2) a GTP-binding pathway involving GTP/GDP.
FIG. 4A is a schematic view of a representative phosphorylation pathway. Phosphorylation is the predominant mechanism used to regulate protein activity in eucaryotic cells. In phosphorylation, a phosphate group (P) is reversibly attached to the side chain of an amino acid in a protein. The attached phosphate group may cause structural changes in the protein, for example, due to electrostatic interactions between the negative charges on the phosphate group and positive charges on the side chains of nearby amino acids. These structural changes may affect the activity of the phosphorylated protein, enhancing or inhibiting its function.
Specialized enzymes control phosphorylation in cells. In particular, protein kinase enzymes transfer phosphate groups to proteins, and protein phosphatase enzymes remove phosphate groups from proteins. Protein kinases and protein phosphatases are found in great variety in eucaryotic cells: a single cell may contain more than 100 different kinases, and one percent of genes may code for kinases.
There are two major categories of kinases: (1) serine/threonine (S/T) kinases, and (2) tyrosine kinases. The S/T kinases function by selectively phosphorylating serine and threonine side chains on substrate proteins or peptides. These kinases include cyclic AMP-dependent kinase (A-kinase), cyclic GMP-dependent kinase (G-kinase), protein kinase C(C-kinase), Ca2+-calmodulin-dependent kinase (CaM-kinase), phosphorylase kinase, MAP kinase, and TGF-β receptor, among others. The S/T kinases are predominantly cytosolic. The tyrosine kinases function by selectively phosphorylating tyrosine side chains on substrate proteins or peptides. These kinases include the receptor kinases for epidermal growth factor (EGF), platelet-derived growth factor (PDGF), fibroblast growth factors (FGFs), hepatocyte growth factor (HGF), insulin, insulinlike growth factor-1 (IGF-1), nerve growth factor (NGF), vascular endothelial growth factor (VEGF), and macrophage colony stimulating factor (M-CSF). These kinases also include the nonreceptor kinases associated with the tyrosine-kinase-associated receptors, such as the Src family (Src, Yes, Fgr, Fyn, Lck, Lyn, Hck, and Blk) and Janus family (JAK1, JAK2, and Tyk2) kinases. The tyrosine kinases are predominantly membrane bound. A few kinases function by selectively phosphorylating threonine and tyrosine side chains on substrate proteins or peptides. These kinases include the mitogen-activated protein (MAP) kinase-kinase.
FIG. 4B is a schematic of a representative GTP-binding pathway. The GTP-binding pathway generally resembles the phosphorylation pathway in that each pathway involves transfer of a phosphate group to a protein. However, in the GTP-binding pathway, the protein gains a phosphate group by exchanging a bound GDP for a bound GTP, whereas in the phosphorylation pathway, the protein gains a phosphate group by covalent addition of the phosphate group to a serine, threonine, or tyrosine by a kinase enzyme. The binding of a GTP to a GTP-binding protein may cause structural changes in the protein that in turn affect the activity of the protein. Examples of GTP-binding proteins include the trimeric G-proteins described above and the Ras superfamily of monomeric GTPases. The Ras proteins are activated by release of bound GDP and binding of GTP stimulated by guanine-nucleotide releasing proteins (GNRPs). The Ras proteins are inactivated by hydrolysis of the bound GTP by GTPase-activating proteins (GAPs).
A physiological response may require stimulation by only a single type of signal substance, or may require stimulation by two or more types of signal substances. The latter mechanism permits finer tuning of the physiological response through signal integration. For example, a protein may be activated only by phosphorylation by two different kinases, themselves activated by binding of two different signal substances to two different receptors. Alternatively, a protein may be activated only by concurrent phosphorylation and GTP binding, or by binding of two subunits whose binding is contingent on phosphorylation by separately activated kinases.
3. Effects of Errors
Errors in the signal transduction and regulation pathways described above can cause cancer and other diseases. Indeed, a primary cause of cancer is a mutation that makes a stimulatory gene product hyperactive, converting a proto-oncogene into an oncogene. The primary classes of known proto-oncogenes include the following cell-signaling proteins: (1) growth-factor receptors acting via tyrosine kinases, (2) GTP binding proteins, (3) membrane/cytoskeleton-associated tyrosine kinases, (4) cytoplasmic tyrosine kinases, (5) steroid-type growth-factor receptors, and (6) S/T kinases. Consequently, cell-signaling proteins have become important subjects of research and drug development.
Assays that determine the presence and activity of cell-signaling components are important tools for high-throughput screening laboratories. Unfortunately, current assays have a number of shortcomings. For example, the presence and activity of kinases can be determined using assays capable of detecting phosphorylated amino acids. In a standard kinase assay, radioactive ATP and an appropriate protein substrate are added to a sample. If the sample includes kinases, radioactive phosphate groups will be transferred from the radioactive ATP to the protein substrate. The protein substrate and radioactive ATP can be separated, and the presence and activity of kinases determined by assaying the amount of radioactive protein substrate. Unfortunately, this assay involves radioactivity, presenting a short-term safety hazard for the assay operator and a long-term storage and disposal problem. Moreover, the assay is heterogeneous, requiring separation of components for analysis. Significantly, assays for other cell-signaling components may have similar shortcomings, as well as slow time courses and unstable endpoints that require precise timing of assay readouts. Thus, there is a need for improved assays for detecting the presence and activity of cell-signaling components.